Epidemiological studies on periodontal diseases conducted in many countries employing different indices have generally shown advanced periodontal destruction to be more prevalent in adult males than in adult females. The 1984 Hong Kong survey of adult oral health, using the Community Periodontal Index of Treatment Needs (CPITN) to determine periodontal status, revealed that in males aged 35 to 44 years the prevalence of deep pockets was 23 per cent, compared with 9 per cent in females (P less than 0.01). Males had a mean of 0.4 sextants with deep pockets whereas females had a mean of only 0.1 sextants so affected (P less than 0.01). Females were found to have a significantly larger proportion of healthy sextants (P less than 0.01). The observed sex difference in disease prevalence may have been influenced by the fact that fewer teeth were present in the females, but it could not be explained by differences in reported oral hygiene measures and practices.